1. Field of the Invention
The invention is generally related to detection and sensing of properties of downhole fluids. More particularly, this patent specification relates to downhole spectroscopic detection of substances such as Hydrogen Sulfide using colloidal detection mixtures.
2. Background of the Invention
Hydrogen sulfide (H2S) occurs extensively in a number of subsurface hydrocarbon reservoirs under anaerobic conditions. The presence of hydrogen sulfide is highly corrosive to casing, tubing, and other metallic and polymeric tools, an effect that is considerably accelerated by low pH and the presence of carbon dioxide. This has a significant impact on the overall hydrocarbon recovery processes, during which materials selection and corrosion control are of great importance. Additionally, H2S is hazardous to humans even at minute concentration levels (for example, about 100 ppm).
The H2S content of reservoir fluids can be determined from samples collected by wireline fluid Sampling tools such as Schlumberger's Modular Dynamics Tester or other sampling tools. Fluid samples are usually collected in metal containers, which are able to maintain the pressures at which the samples were collected. However, a problem associated with sampling fluids containing hydrogen sulfide is partial loss of the gas by reaction of the metal components, particularly those made from iron-based metals. The hydrogen sulfide gas readily forms non-volatile and insoluble metal sulfides by reaction with many metals and metal oxides, and analysis of the fluid samples can therefore give an underestimate of the true sulfide content.
As a result, the in situ detection and measurement of hydrogen sulfide is widely regarded as a critical parameter needed for well completion and production strategies. Due to the high chemical reactivity of sulfide species, various detection strategies including spectroscopy, electrochemistry, chromatography and combinations thereof have been proposed. For example, see Wardencki, W. J. “Problems with the determination of environmental sulphur compounds by gas chromatography” Journal of Chromatography A, Vol 793, 1 (1998). U.S. Pat. No. 6,939,717B2 describes feasible electrochemical and optical methodologies and embodiments aimed at downhole detection of hydrogen sulfide.